Underwater acoustic positioning system and method

ABSTRACT

A method for determining the position of an underwater device includes placement of a plurality of station keeping devices on or below the surface of the water in known positions. A device to locate is provided for placement below the surface of the water, and the device to locate and the station keeping devices are provided with a synchronized time base and a common acoustic pulse time schedule. Each station keeping device sends an acoustic pulse at a time according to the common acoustic pulse schedule. The device to locate receives pulses sent by the station keeping devices and calculates a distance between itself and each station keeping device based upon the time that the acoustic pulse is sent and the time that the pulse is received. The device to locate then calculates its position based upon the distances between the device to locate and the station keeping devices. Systems and devices are also disclosed.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/708,741, filed on Aug. 16, 2005, and entitled “UnderwaterAcoustic Positioning System,” which patent application is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to systems for locating the position of an objectand, in particular, to an acoustic system for locating the position ofan object located underwater.

BACKGROUND

A system that accurately determines the position of an underwater devicewould be highly useful for a number of underwater activities. GPS canprovide very accurate position location information on the surface ofthe globe. GPS refers to the Global Positioning System, a constellationof more than two dozen GPS satellites that broadcast precise timingsignals by radio to electronic GPS receivers which allow them toaccurately determine their location (longitude, latitude, and altitude)in real time. GPS receivers calculate their current position (latitude,longitude, elevation), and the precise time, using the process oftrilateration after measuring the distance to at least four satellitesby comparing the satellites' coded time signal transmissions. However,since radio waves at the frequency of GPS attenuate very quickly inseawater, the radio-based system cannot be directly used underwater.

Acoustic transmission works very well in water, with low losses, at asound velocity of approximately 1500 meters/second. As a result,acoustic-based positioning systems have generally been used instead ofGPS or other radio-based systems for underwater positioningdetermination.

One type of prior art system for underwater positioning is known as LongBase Line acoustic positioning (LBL). In most LBL schemes, the Device toLocate (DTL) is active and pings when it receives a sound. A signalsending device sends an acoustic signal to activate the DTL, and senderthen receives the response ping and determines the time (round tripdelay) to the DTL. The roles can be reversed, but no global timingdomain is needed underwater. Where devices are located on the surface ofthe water, the surface devices can use GPS to get a spatial fix.

An LBL system typically has two elements: the first element includes anumber of transponder beacons moored in fixed locations on the seabed(or, for example, on buoys fixed to the sea bed), and the second elementconsists of an acoustic transducer in a transceiver that is temporarilyinstalled on a vessel or tow fish. The positions of the beacons aredescribed by a coordinate frame fixed to the seabed, and the distancesbetween them form the system baselines. The distance from a transponderbeacon to the transceiver is measured by causing the transducer totransmit a short acoustic signal that the transponder detects and thenresponds to by transmitting an acoustic signal. The time from thetransmission of the first signal to the reception of the reply signal isthen measured. Since sound travels through water at a known speed, thedistance between the transponder beacon and the transducer can then beestimated. The process is repeated for each of the remaining transponderbeacons, allowing the position of the vessel relative to the array ofbeacons to be calculated or estimated.

In principle, navigation can be achieved using just two seabedtransponder beacons. In such a case, however, which side of the baselinethe vessel is located on may be ambiguous. In addition, the depth orheight of the transducer must be assumed (or separately measuredaccurately). Three transponder beacons is therefore the minimum requiredfor unambiguous navigation in three dimensions and four is the minimumrequired if redundancy is desired to allow for checks on the quality ofnavigation. The LBL system works very well, but requires that both thebuoys and DTLs transmit data increasing the amount of acoustic bandwidthrequires as the number of DTLs are increased, limiting the amount ofDTLs that can participate in the scheme. The complexity and powerconsumption of the DTLs system is increased significantly as it mustalso transmit data to the buoys.

Another type of prior art system for underwater positioning is known asShort Base Line (SBL) positioning. An SBL system is normally fitted to avessel, such as a barge, semi-submersible, or a large drilling vessel. Anumber of acoustic transducers are fitted in a triangle or rectangle onthe lower part of the vessel. There are at least three transducers, butthe typical number is four. The distance between the transducers (thebaselines) are caused to be as large as is practical, typically aminimum of 10 meters. The position of each transducer within aco-ordinate frame fixed to the vessel is determined by conventionalsurvey techniques or from an “as built” survey of the vessel.

SBL systems transmit from one, but receive on all transducers. Theresult is one distance (or range) measurement and a number of range (ortime) differences. The distances from the transducers to an acousticbeacon are measured in a manner similar to what has been described forthe LBL system, allowing the position of the beacon to be computedwithin the vessel co-ordinate frame. If redundant measurements are made,a best estimate can be calculated that is more accurate than a singleposition calculation. If it is necessary to estimate the position of avessel in some fixed, or inertial, frame, then at least one beacon mustbe placed in a fixed position on the seabed and used as a referencepoint.

With an SBL system, the coordinate frame is fixed to the vessel, whichis subject to roll (change in list), pitch (change in trim) and yaw(change in heading) motion. This problem must be compensated for byusing additional equipment such as a vertical reference unit (VRU) tomeasure roll and pitch and a gyrocompass to measure heading. Thecoordinates of the beacon are then transformed mathematically to removethe effect of these motions. The SBL system suffers the same problems asthe LBL system, namely; required underwater bandwidth increases linearlyas you add more devices to locate, increased complexity of DTLhardware/software and very difficult setup of the system.

The terms Long Base Line and Short Base Line are used because, ingeneral, the baseline distances are much greater for Long Base Line thanfor Short Base Line positioning (and even Ultra-Short Base Linepositioning; USBL). Because the baselines are much longer, an LBL systemis more accurate than SBL and USBL. LBL also has the advantage ofpositioning the vessel or other object directly in a fixed, or inertial,frame. This eliminates most of the problems associated with vesselmotion. In all these systems, the array of seabed beacons needs to becalibrated. There are several techniques available for achieving this,with the most appropriate one being dependent on the task and theavailable hardware.

There are other prior art systems that provide underwater positiondetermination, including that disclosed in U.S. Pat. No. 5,119,341 toYoungberg, entitled “Underwater GPS System.” The Youngberg scheme isessentially the direct transposition of GPS coding techniques forunderwater use, wherein satellites are replaced by buoys and radar wavesare replaced by acoustic waves traveling from the buoys to underwatermobiles. The equipment on board the underwater body has an architecturethat is very similar to the one encountered in a GPS receiver. Astabilized clock is used for accurate measurement of the time of arrivalof the acoustic pulses transmitted sequentially by the buoys. Knowingthe velocity of sound in water, it is then possible to calculate thedistance to the buoys.

The Youngberg methodology is a full GPS-like scheme, wherein the surfacebuoys send coded information similar to that sent by the GPS satellites.The device underwater keeps a stabilized clock, compares the arrivaltime of the start of a message, and uses the time sent data in themessage along with the buoy location. A disadvantage of this scheme isthat it relies on receiving some very long messages in the noisyunderwater environment. The Youngberg system also has the buoys freefloating or moving and sends position data on the buoys location on aregular basis. This results in even further complexity in the datamessage, wherein the time of the start of the data, along with the TIMESENT and LOCATION OF BUOY message data, is required to locate position.It can be very difficult to send high bandwidth acoustic data in thenoisy ocean environment, making this approach difficult in practice. Ina practical sense the Youngberg scheme requires that the DTLs have aclear data channel from each of the buoys down to the DTLs using anacoustic modem or they can't identify their position, in the underwateracoustic environment this is very difficult to achieve.

The GPS Intelligent Buoys scheme (GIB) is disclosed in U.S. Pat. No.5,579,285 to Hubert, entitled “Method and device for the monitoring andremote control of unmanned, mobile underwater vehicles.” This systemuses a scheme wherein buoys on the surface listen for data sent up fromthe DTL. Other data is sent back down from the buoys. It is similar tothe Youngberg scheme, but instead uses upward acoustic flow of data. Thetracking principle is based on measuring the time of arrival at a set ofbuoys of an acoustic pulse sent by the DTL at a known time. At a regularinterval, each buoy transmits to a processing center its D-GPS positionand the time of arrival of the acoustic pulses. Knowing the soundvelocity, distances from the buoys to the DTL are easily calculated. Theminimum number of buoys to deploy is two, as there are only twounknowns, the mobile's depth being sent upwards using a telemetrychannel. Some small number of mobiles can be tracked together using timeor frequency diversity. The GIB system is limited by the fact that eachunderwater device to locate (DLT) must use some of the limitedunderwater acoustic communication channel to send acoustic data to thesurface buoys, and therefore the number of devices being tracked islimited to a small amount.

SUMMARY OF THE INVENTION

The invention provides systems, methods, and devices that allow a deviceto locate to determine its position underwater. The invention canprovide robust and reliable position calculations that, in exemplaryembodiments, are accurate within one meter despite noisy underwaterconditions, even where several devices to locate are deployed with thesystem.

In a first aspect of the invention, a method for determining theposition of an underwater device is provided. The method includesplacement of a plurality of station keeping devices on or below thesurface of the water in known positions. A device to locate is providedfor placement below the surface of the water, and the device to locateand the station keeping devices are provided with a synchronized timebase and a common acoustic pulse time schedule. Each station keepingdevice sends an acoustic pulse at a time according to the commonacoustic pulse schedule. The device to locate receives pulses sent bythe station keeping devices and calculates a distance between itself andeach station keeping device based upon the time that the acoustic pulseis sent and the time that the pulse is received. The device to locatethen calculates its position based upon the distances between the deviceto locate and the station keeping devices.

In a further aspect of the invention, a system for determining theposition of an underwater device to locate is provided. The systemincludes a plurality of station keeping devices on or below the surfaceof the water. Each station keeping device has a means for determiningits position, a means for keeping time, a transducer for generatingacoustic pulses in the water, a memory for storing a schedule ofacoustic pulses, and a processor in communication with the memory andthe transducer for directing the generation of acoustic pulses accordingto the schedule. At least one device to locate is also provided. Thedevice to locate includes a means for keeping time, a memory for storingthe schedule of acoustic pulses and a known position for each stationkeeping device, an acoustic transducer for receiving pulses sent by thestation keeping devices, and a processor configured to calculate theposition of the device to locate underneath the water based upon thearrival time of the acoustic pulses. Each means for keeping time in thesystem is synchronized and each memory stores the same schedule ofacoustic pulses.

In a still further aspect of the invention, a device to locate that cancalculate its position underwater in a system having a plurality ofstation keeping devices on or below the surface of the water isprovided. Each station keeping device in the system is located in aknown position and includes a means for keeping time that issynchronized with a system time base, a transducer for generatingacoustic pulses in the water, a memory for storing a common systemschedule of acoustic pulses, and a processor in communication with thememory and the transducer for directing the generation of acousticpulses according to the schedule. The device to locate includes a meansfor keeping time that is synchronized with the system time base, amemory for storing the common system schedule of acoustic pulses and theknown position for each station keeping device, an acoustic transducerfor receiving pulses sent by the station keeping devices, and aprocessor configured to calculate the position of the device to locateunderneath the water based upon the arrival time of the acoustic pulses.

The present invention provides a significant advantage over the priorart systems in that it can use very little acoustic bandwidth and theamount of data is fixed for any number of devices to locate. In thepresent invention, the signal from each buoy arrives at a known time andhas predetermined data (for example, an acoustic ping at a knownfrequency and duration). The signal is very easy to detect in noisyenvironments and it leaves most of the underwater acoustic channelavailable for other uses such as acoustic data modems. In addition, bycalculating acoustic attenuation of the locating signals based ondistance and adjusting a variable gain amplifier to account for theattenuation, the positioning systems and methods can be made even morerobust.

The position determination by a device to locate in the presentinvention doesn't require that the underwater device transmit anyacoustic data, it can simply listen to the transmission from the stationkeeping devices. This allows the hardware to implement these systems andmethods to be very simple, inexpensive, and compact as it is notrequired to transmit data out from the device. This scheme is alsocovert where the device doesn't have to transmit acoustic energy toderive it's location.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings:

FIG. 1 illustrates a system of the invention having three surface buoyssending timed pulses to a device to locate;

FIG. 2 illustrates a geometrical solution to the problem of locating apoint given the distance from three known points, as employed in thepresent invention;

FIG. 3 provides a flow chart illustrating the operation of a stationkeeping device of the invention;

FIG. 4 provides a block diagram illustrating exemplary electronics foroperation of a station keeping device of the invention;

FIG. 5 provides a flow chart illustrating the operation of a device tolocate of the invention;

FIG. 6 provides a block diagram illustrating exemplary electronics foroperation of a device to locate of the invention;

FIG. 7 illustrates a device to locate of the invention having a display;and

FIG. 8 illustrates mission planning software useful with systems andmethods of the invention.

DETAILED DESCRIPTION

The present invention provides a method and apparatus for locating anaccurate 3-dimensional position of an underwater device or vehicle usingan acoustic signaling method and a stabilized time base. The stabilizedtime base can be provided on the device to locate using a stabilizedclock. If the depth of the device to locate (DTL) is known, the systemrequires at least two spatially separated acoustic transmitters (buoys)in fixed locations on the surface of the water. These buoys are referredto as station keeping buoys. An alternative scheme would allow the buoysto be fixed underwater at a known location, not subject to the positionerror when they are floating, the time base for such a station keepingbuoy could be provided by a floating GPS receiver on a cable (GPS timewould only be available through the cable) or a stabilized clock such asused in the DTLs. The time on the buoys could also be updated by cable,by coming to the surface from time to time, or some other acousticscheme that allows it to correct for time drift.

The buoys and any number of underwater receivers (DTLs) have veryaccurate clocks that are synchronized (i.e., the system has a stabilizedtime base). The buoys at known locations can send pings at known timesbased on a shared schedule. The receivers underwater can know the timethat the acoustic pulse (ping) should arrive, as well as the pulseamplitude, and can therefore measure the time of arrival veryaccurately. Where the system uses a single ping from each buoy and knowsthe time of arrival very accurately (since its position is very wellpredicted from the last fix), the system can easily identify the signalin a noisy environment, making the system very robust.

A system 10 of the invention is illustrated in FIG. 1. In theillustrated embodiment, three buoys 12 are provided on the water surface14. The buoys 12 are generally stationary, and their positions can bestabilized using station keeping compulsion or by being anchored to theseabed 16. The buoys 12 are illustrated on the water surface 14,however, in certain embodiments, they can also be placed below the watersurface 14 in known or knowable positions. The buoys 12 include acoustictransducers 18 that can send timed acoustic pulses 20 that can bereceived by device-to-locate 22 having an acoustic transceiver 26 at adepth D below the water surface 14. In this embodiment, buoys 12 anddevice-to-locate 22 maintain a stabilized time base. While three buoys12 are illustrated, it may be possible to use fewer, depending upon thegoals for determining the position of device-to-locate 22, and could bemore than three for a number of reasons, including for redundancypurposes or increased range of position determining capability. Onedevice-to-locate 22 is illustrated, but any number can be provided, andone advantage that can be provided by the present invention is theability to determine the position of large numbers of such devices.

In illustrated embodiment, the buoys 12 use a GPS receiver 24 to keepthem in a synchronized time domain. The DTLs 22 can have very accuratetemperature-stabilized oscillators that are synchronized and calibratedto the common UTC time domain so that when the devices go underwater,they can also maintain the stabilized time base. Every device 12,22 inthe system 10 can thus be on a common time base and have a commonschedule of acoustic pulses 20. The buoys 12 that transmit acousticlocator pulses 20 can start transmitting simple pings (short burst ofacoustic energy, ringing the transducer) on a schedule (using GPS time)from known locations.

The schedule on which the acoustic pulses 20 are based can be a periodicschedule for simplicity, and separated in time so that no two buoys'signals collide in the field of interest where the device-to-locate 22receivers 26 operate. The underwater devices can know their startinglocation when they first go underwater, using a GPS fix, and cantherefore time the arrival of each pulse (knowing the time it was sentand the location of the buoy) and derive an approximate distance to eachbuoy (speed of sound multiplied by time). Since the DTL knows thedistance to each buoy very closely, it can use this fact to ignore pingsor acoustic noise in all time windows except the very small window oftime when the actual pulse would arrive. The DTL can include anadjustable gain amplifier and can adjust the gain to have the signalstrength at a uniform level. In one embodiment, the pulse is a veryshort duration signal and only the pulse that arrives first (i.e. theshortest path) is of interest, any longer path reflections can beignored. In this embodiment, the pulse can be a simple ping, i.e., aknown signal that has no variation or information contained, so thatonly its arrival time needs to be determined in order for the DTL todetermine its location. While other data may be transmitted or broadcastby the buoys, it is not required by the DTL to determine its position.

The method of the present invention uses short acoustic signalstransmitted on a schedule and is very low bandwidth and requires only aone way signal transmission in order for the DTL to determine itsposition. The DTL can listen for a very simple known signal arriving ata known small window of time. This signal can be of a high amplitude(Sound pressure level) that can be transmitted great distances and isvery noise immune. The ping may have reflected signals that arrive at alater time than the direct ping, but these can be ignored since thefirst pulse travels the shortest distance and is therefore is the firstone measured and timed by the DTL. In fact, the ping method is what isused to accurately measure the depth of water under a boat on the ocean.This technique can therefore be used at great depths and can measure thereflected signal in very high noise environments.

The present invention can use low frequency pings, since the pings neednot carry any information. The signal is a known value, similar to a “1”in binary. When transmitting data, the signal is unknown, so noise cancause confusion as to the received value (“0” or “1”). In the case ofthe timing ping, the onset of the signal is all that is used to measurethe travel time. Use of a lower frequency allows the signals to travelmuch further with lower losses.

In a preferred embodiment, the schedule of pulse 20 transmission is setto be quick enough to keep the position updates regular, but spaced longenough apart to avoid confusion in distinguishing the first pulse onsetfrom each buoy from other signals or reflections. The transducerfrequency of each buoy can also be different, so as to allowdiscrimination between buoys in the same time space, or the samefrequency may be used but separated in time. The schedule can also bedesigned knowing the propagation speed, so that if there are areas wheretwo buoys would conflict, i.e. the signal arrival would be at the sametime given the transmission time, the schedule can be alternated toguarantee that the second sequence will be separated in arrival time.

Three exemplary Transmission Schedules are presented in Table 1 below.The times shown for these alternative schedules are the times at whichthe buoys 12 transmit a pulse or ping 20. The device to locate 22 willobtain the arrival time of each ping and, knowing the transmission timeand having a common global clock domain, can then calculate the distanceto each buoy 12.

TABLE 1 Transmission Schedule 1: Period: 60 seconds Buoy1: T = 0, 3, 6,. . . 57, f = 30 khz Buoy2: T = 1, 4, 7, . . . 58, f = 30 khz Buoy3: T =2, 5, 8, . . . 59, f = 30 khz Transmission Schedule 2: Period: 60seconds Buoy1: T = 0, 10, 30, 40, f = 100 khz Buoy2: T = 5, 20, 35, 50,f = 100 khz Buoy3: T = 10, 25, 40, 55, f = 100 khz Transmission Schedule3: Period: 60 seconds Buoy1: T = 0, 3, 6, . . . 57, f = 30 khz Buoy2: T= 0, 3, 6 . . . 57 f = 32 khz Buoy3: T = 0, 3, 6, . . . 57 f = 34 khz

Transmission Schedule 2 alternates the second transmission, in order toallow for arrival time separation. In these examples, the cycle may berepeated every minute. Transmission Schedule 3 has the pulsestransmitted at the same times, but has frequency differences for eachbuoy so that the signals can be distinguished even if they have the samearrival time at a given point in space.

If the device-to-locate 22 knows its depth using a pressure transduceror other device, it is possible to use a system with only two buoys 12.In this case, while there are two solutions for position, since theapproximate solution is known, then the position of the DTL can still beresolved (2 equations, 2 unknowns). With three buoys, X, Y, Z (latitude,longitude and depth) can be resolved, or, if depth is known, one cansolve for a more accurate speed of sound (Distance=time*speed of sound).An example of the geometric solution for trilateration (triangulation)can be found in the example presented in conjunction with FIG. 2 below.

When the system is operated with three our more buoys 12 and thereceivers 26 (on DTLs 22) are at a known depth D, extra data isavailable that can be used to calculate the actual average speed ofsound in the area between the buoys 12 and DTLs 22. Using trilateration(triangulation), a simple calculation can yield the latitude, longitude,and depth (X, Y, Z) of the DTL. Using two measurements in time, the DTLcan also calculate the speed and heading it is moving on. Thisinformation can also be used for predicting the time of the next pulse.

FIG. 2 pictorially depicts the geometrical solution to the problem oflocating a point (i.e., DTL 22) when given its distance (L₁, L₂, L₃)from three known points (i.e., station keeping buoys 12). In thisthree-dimensional coordinate system, the X-axis is defined as the linebetween the first and second reference points. The Y-axis is a lineperpendicular to the X-axis, passing through the first point, in theplane of the three points, and in the direction of the third point. TheZ-axis is defined by a right-handed coordinate system between the X- andY-axes. By connecting the four points with lines, the tetrahedron isformed. In use with the present invention, the tetrahedron wouldgenerally be flipped upside down, with the three known points being thebuoys 12 on the surface of the water and the bottom point being theunderwater device to locate 22.

Using this geometry, the depth and position of the receiver 22 can befound using a few calculations. The following calculations can beutilized in the present invention to find the 3-dimensional position ofa point, given its distance to three known points in 3-dimensionalspace:X=(L ₁ ² −L ₂ ² +L _(B1) ²)/(2L _(B1))C ₁=(L ₁ ² −X ²)^(1/2)X _(B)=(L _(B3) ² −L _(B2) ² +L _(B1) ²)/(2L _(B1))C _(B)=(L _(B3) ² −X _(B) ²)^(1/2)D ₁=(C ₁ ²+(X _(B) −X)²)^(1/2)Y=(D ₁ ² −L ₃ ² +C _(B) ²)/(2C _(B))Z=(C ₁ ² −Y ²)^(1/2)

The geometry may also be extended to more than three points, allowingone to cancel out the errors in length measurements and/or improve thespeed of sound estimation used to calculate the lengths from thetransmitters. In addition, the L_(n) lengths can be converted totime*speed-of-sound and then solved for speed of sound with data from anextra buoy.

The buoy 12 location, although generally or substantially fixed bykeeping it on stationary propulsion devices or by anchoring, may stillchange with wave motion or from tides, currents, or waves pushing it offstation. This error can be partially corrected for if the buoy 12 knowsthe general direction of the receivers 26 (DTLs 22) between the buoys12. For example, if a buoy 12 moves up 10 feet, the buoy can partiallycorrect for this error by advancing the schedule of sending the ping sothat the ping has already traveled 10 feet by the scheduled transmissiontime. Geometry can yield better error fits for adjusting the launchtime, advancing or retarding it in order to help cancel out the errorscaused by the movement of the buoy. In this way, the error can begreatly reduced when the arrival pulse is timed at the DTL.

A basic flow chart for the operation of a buoy 12 is illustrated in FIG.3. Upon Mission Start 40, configuration data can be loaded on 42 ortransmitted to the buoy. The configuration data can include the commonping schedule, or position or time data. The buoy can determine itslocation, typically by taking a GPS fix 44. Where the buoy includespropulsion, it can move to its desired station position 46, checking itsGPS fix along the way to determine when it has reached the stationposition 48. Once the buoy is within some desired range of its stationposition, it can read the time and location through its GPS system 50. Aposition servo can keep the buoy at its station position, and bytracking its position, it can generate the its positional error 52—thedistance and direction from its original position. Based on this error,the buoy can correct its position if needed, or it can adjust its pingtime with respect to the schedule as described above to account for thepositional error 54. Pings can be generated according to the schedule 56with or without corrections. When the mission is completed 58, the buoycan drive home from is station position 60. Otherwise, the buoy cancontinue to read its time and location, and process from step 50. Oncethe buoy reaches home, its mission is complete 62.

FIG. 4 provides a block diagram illustrating the electronic componentsfor one embodiment of a buoy 12 of the invention. The buoy 12 includes amicroprocessor 80 that drives the logical operation of the buoy. In oneexemplary embodiment, the microprocessor 80 can be a PIC microprocessoravailable from Microchip Technology, Inc. of Chandler, Ariz., forexample a PIC18F258 or similar microprocessor. An RS232 input dataprogram station 82 can be provided to communicate with themicroprocessor 80, for example, to load schedule and position data.

A GPS Receiver 84, with associated GPS Antenna 86, are connected to theprocessor 80. In one exemplary embodiment, the GPS Receiver 84 can be aGarmin OEM system GPS15L (available from Garmin International, Inc. ofOlathe, Kans.). This GPS Receiver 84 can also include a highly accurateone-pulse-per-second (PPS) output. The GPS One-Pulse-Per-Second (PPS)output pulse edge can happen in a synchronized fashion across all GPSreceivers in the buoys 12 and is linked to the Atomic Clock domains onthe GPS constellation. The output is highly accurate and is provided forthe use of applications requiring precise timing measurements. It can beused in the present invention, along with an ASCII time output, toprovide a synchronized time base when all devices are above the surfaceof the water. The DTLs synch onto this time when they are above thewater and maintain the time using a temperature-stabilized oscillatorwhen underwater. The GPS PPS signal can be generated after the initialposition fix has been calculated and can continue until power down. Therising edge of the signal is synchronized to the start of each GPSsecond. To obtain the most accurate results, the PPS output can becalibrated against a local time reference to compensate for cable andinternal receiver delays and the local time bias.

Buoy 12 electronics also includes a power source such as battery 88having a regulated DC output to power the microprocessor 80. A highvoltage DC converter 90 can also be connected between the battery 88 anddrive electronics 92, which drive a transducer 94 to create the pingsignals.

Operation of the DTLs following mission start 100 can be described byreference to the basic flow chart provided in FIG. 5. In general, alldata sent from the buoys 12 travels down to one device-to-locate 22, orin further embodiments, a plurality of DTLs, that use the data. Theschedule of transmissions and location of the buoys 12, or any otherconfiguration data, can be provided 102 to the DTLs 22 before theysubmerge, or this data can be sent to the DTLs 22 on a time schedule.Since this type of data can be difficult to send to all areas of a watercolumn, one embodiment of the present invention doesn't require thatthis information be sent at all, but it can instead be relayed to theDTLs ahead of time, e.g., before the DTLs are placed under water. It isalso possible to send this data at a very low data rate along with theacoustic pulse positioning data on some predetermined schedule to theDTLs while submerged using a modulation scheme like an acoustic modemwould use. This allows updating the devices with a new schedule whileoperating if the buoys change. The data can be designed to integrateinto the schedule after the pings from a receiver. A few bits of dataare sent after each ping, providing the complete data set after somenumber of pings.

Typically before being placed in the water, the DTL can get a GPSpositional fix 104 to determine its current location, and synch to thetime domain of the buoys 12, for example, using the PPS methodologydescribed above. The DTL can then keep time 106, for example, by using astabilized oscillator, the output of which has been synchronized to theGPS time provided by the PPS above. Upon placement in the water, anupdated positional fix based on GPS can be obtained 108 and the distanceto all buoys calculated 110.

If data on position and schedule is required to be sent to the DTLsafter they have been deployed under water, it can be “delta” or changedata such as the change in position from some local datum. This couldbe, for example, feet from a reference point (latitude and longitude ofreference point). This allows sending of a small number of bits, sayunder 32, for each buoy position. Since this data is fairly static, itcan be sent with some very robust coding scheme, such as, for example,pulse on a clock at 100 baud, which can be sent very long distances andis very simple to decode.

Once underwater, the DTL can sort the ping schedule to determine thenext expected ping arrival time 112. The DTL then listens for a pulse ata known time 114 from a given buoy. Since the DTL knows the distance andtime to the buoy, it can watch the window very closely and listen forwhether the pulse was transmitted (pulse present=1, absent=0).

When updating the positional fix, the last position and direction isknown, and the depth and speed can therefore known very accurately.Based on these, the time when the next position pulse will be receivedand how far it will travel can also be predicted very accurately. Sincethe attenuation of the acoustic pulse vs. distance of the medium (e.g.seawater) is also known, one can set the gain on a variable gainamplifier 112, listen for the pulse, and then use the amplitude andarrival time of the pulse to filter out unwanted noise signals. Thisfeature makes this positioning scheme very robust and noise immune andcan be accomplished by adjustment of a very stable LogarithmicAmplifier, such as the AD8330 or AD602 amplifiers available from AnalogDevices, Inc. of Norwood, Mass. By establishing an expected signalarrival time window, even reflected signals that arrive later can berejected as being out of the likely time window. If they happen to fallonto another pulse, they can be distinguished by having weaker amplitudethan the actual first arrival signal. If they collide exactly, thereceived signal will merely be stronger. Since the vehicle speed andposition is also known from difference between the last two measurementsit can be dead-reckoning between calculations so that the position canbe reported at a much higher rate. In one embodiment, systems of theinvention provide geo-referenced data such as Latitude and Longitude anddepth in a given frame of reference such as WGS 84, but it is possibleto output position in any reference frame.

If signals are received and the arrival times stored 116 from all activebuoys 12, processing can continue 118 with further position, directionand velocity values being calculated based on triangulation from thebuoys 120 and outputted 122. The position, direction, and velocityinformation can then be used to calculate the next signal time 124. TheDTL can then return to waiting for the next signal arrival 112. In thecase where signals are not received from each active buoy 118, again,the DTL can return to waiting for the next signal arrival 112.

Referring now to FIG. 6, a block diagram representing the electronicsfor a DTL 22 of the invention is illustrated. The DTL can include amicroprocessor 140, GPS Receiver 144, GPS antenna 146, and battery 148similar to those provided for the buoy 12 electronics described above. Astabilized oscillator 150 is included to provide for maintaining timesynchronization to the GPS atomic clock when the device is workingunderwater. The oscillator 150 could be, for example, a FOX Model FPC5AF Series 50.8 mm×50.8 mm oven controlled crystal oscillator, or anyother suitable device known in the art. The standard versions of such anoscillator can provide a very low drift of 2 parts per billion whenwarmed up to their operating temperature after 3 minutes. In thepreferred embodiment, this clock may operate at 10 MHz and provide theclock signal for the microprocessor 140. The 10 MHz clock is timed andsynchronized to the PPS GPS clock when above water. The stability andaccuracy of the oscillator is compared to the GPS clock while the deviceis above water and any frequency errors can be adjusted out of thesystem with a correction factor, such as add 5 nanoseconds every second,etc. A low drift clock that is synchronized to the GPS clocks on thebuoys, combined with a schedule, can allow the DTL to know apriori whenthe acoustic data is sent can be an important feature of the devices ofthe invention. This allows use to time the signal transit time of theacoustic pulse and derives the distance traveled. In a system with 2 ppbof frequency error or better, this can work out to a timing error of0.86 milliseconds over a 12 hour period underwater, this relates todistance error of 0.13 meter measurement error of the distance from thebuoys. This allows for a very accurate sub meter measurement of theunderwater position over a useful mission time. More accurate timing(currently available commercial devices have 0.5 or lower ppb offrequency error) will result in more accurate position resolution.

A variable gain amplifier 152, as described above, can be connected tothe microprocessor 140, and also to a transducer 154 that receives theacoustic signals from the buoys 12.

In the illustrated embodiment, the electronics for the devices can bevery small, something on the order of less than 2 inches square by 0.4inches thick. The GPS active antennas are on the order of 1 inch×1 inch.The present invention can be implemented entirely using components knownin the art, components that are typically available at low cost. Thepreferred embodiment uses a standard GPS OEM receiver PPS output and astabilized oscillator tied to a PIC microprocessor. Commerciallyavailable GPS synch clock devices suitable for use with the presentinvention include the GPSClock Model 200 and the Accord GPS ClockNAV2300R-TD1, but any commercially available or specifically developeddevice known to or devisable by someone of skill in the art would besuitable.

In one embodiment, the device-to-locate can be carried by a diver. Forexample, the electronics could be placed in a device that could be wornlike a wrist watch (using strap 214), and that provides position, depth,speed, distance and/or direction information to the diver. A diagram ofsuch a device 200 is provided in FIG. 7. A display 202 is provided onthe device that can illustrate, for example, the direction 204 and 206distance to a target 208—in the illustrated embodiment, the target is“home.” A push button 212 can be provided to change the current functionof the device, for example by providing direction home or to some othergoal, current position, speed and heading, other modes, or to activate abacklight.

Device-to-locate 200 also includes an acoustic transducer element 210.The transducer can be receive only, or it can receive and send acousticpulses. A signal strength indicator 216 can give the wearer a sense forhow well acoustic pulses are being received. The send or transmitfeature could be used to send data to buoys 12 or other devices, or tosend a reply ping when a ping is received from a buoy. A reply pingwould allow the buoys to determine the position of the DTL (based on thereturn trip time from the original pulse being sent to the reply pulsebeing received) as well as allowing the DTL to determine its ownposition. If sending data, the transmissions could be based on timeslots. The device 200 could also be configured to broadcast an amessage, for example, based on the position of the device or to providefor an emergency message to be broadcast by a diver. If the diver hadstrayed too far from the buoys for example, the device could flash analarm for the diver, and also alert the buoys. A variety of othersensors could also be integrated into the device, and could beconfigured to trigger alarms. For example, a heart rate sensor, a bloodoxygen level sensor, or a tank pressure sensor could be integrated withthe device.

In addition, the device to locate could comprise an unmanned underwatervehicle (UUV) or autonomous underwater vehicle (AUV). FIG. 8 provides ascreen shot of a vector map software system that allows mission planningfor, e.g., an AUV. The Figure shows the location on a map 300 of threebuoys 14. The planning software can be used to layout a desired path forone or more AUVs to follow in carrying out a mission. In this case, afirst AUV path 302 and a second AUV path 304 will direct a first AUV anda second AUV, respectively, on back and forth paths appropriate for anundersea mapping mission. Each AUV can be loaded with at least a map ofits own mission, and can navigate along its planned path by calculatingits positions based on acoustic pulses received from the buoys asdescribed above. The AUVs can further calculate their absolute speed anddirection as described above, to further ensure that the AUVs remain ontheir predetermined courses while performing tasks such as mapping,searching, or security monitoring.

A person of ordinary skill in the art will appreciate further featuresand advantages of the invention based on the above-describedembodiments. For example, specific features from any of the embodimentsdescribed above may be incorporated into devices or methods of theinvention in a variety of combinations and subcombinations, as well asfeatures referred to in the claims below which may be implemented bymeans described herein. Accordingly, the invention is not to be limitedby what has been particularly shown and described, except as indicatedby the appended claims or those ultimately provided. Any publicationsand references cited herein are expressly incorporated herein byreference in their entirety.

1. A method for determining the position of an underwater device,comprising: providing a plurality of station keeping devices on or belowthe surface of the water, each station keeping device being located in aknown position; providing a device to locate below the surface of thewater; providing each of the station keeping devices and device tolocate with a synchronized time base; providing an acoustic pulse timeschedule to each of the station keeping devices and the device tolocate; sending an acoustic pulse at a time according to the acousticpulse schedule from each one of the station keeping devices, the pulsebeing received by the device to locate; calculating a distance betweenthe device to locate and each station keeping device based upon thescheduled time that the acoustic pulse is sent and the time that thepulse is received; and calculating a position of the device to locatebased upon the distances between the device to locate and the stationkeeping devices.
 2. The method of claim 1, wherein calculating aposition of the device to locate comprises a triangulation calculation.3. The method of claim 1, wherein the time that the acoustic pulse isreceived is determined by detecting the onset of the acoustic pulse. 4.The method of claim 3, wherein the acoustic pulse is a simple ping. 5.The method of claim 1, wherein the device to locate calculates a likelytime window for the receipt of an acoustic pulse based at least upon acalculated position of the device to locate and the device to locatelistens for the acoustic pulse during the likely time window.
 6. Themethod of claim 1, wherein the device to locate calculates a velocityand direction of motion of the device to locate based upon at least twocalculated positions of the device to locate.
 7. The method of claim 6,wherein the device to locate calculates a likely time window for receiptof an acoustic pulse based at least upon a calculated position of thedevice to locate and the calculated speed and direction of the device tolocate and the device to locate listens for an acoustic pulse during thelikely time window.
 8. The method of claim 1, wherein the device tolocate calculates an acoustic pulse attenuation value and adjusts avariable gain amplifier to account for the calculated attenuation of theacoustic pulse.
 9. The method of claim 1, wherein the device to locatemeasures a local speed of sound value by measuring the time between ascheduled sending of an acoustic pulse by a station keeping device andreceipt of the pulse by another device over a known distance and thelocal speed of sound value is used by the device to locate issubsequently used to calculate the distance between the device to locateand a station keeping device.
 10. The method of claim 1, wherein thestation keeping devices are buoys.
 11. The method of claim 10, whereinthe buoys are subject to movement with respect to their known position,and the buoys adjust the timing of the transmission of an acoustic pulsewith respect to the acoustic pulse time schedule to account for themovement.
 12. The method of claim 10, wherein the buoys determine theirown position using a GPS receiver.
 13. The method of claim 12, whereinthe synchronized time base is provided using GPS PPS signals generatedby the buoys.
 14. The method of claim 1, wherein, upon receipt of theacoustic pulse by the device to locate, the device to locate sends areturn pulse that is received by the station keeping devices and thestation keeping devices calculate a position of the device to locatebased at least upon the time at which the return pulse is received. 15.The method of claim 1, wherein an acoustic data communication channel isestablished between the device to locate and the station keepingdevices.
 16. The method of claim 15, wherein the acoustic datacommunication channel is used to transmit data at a time provided fordata transmission provided by the acoustic pulse schedule.
 17. Themethod of claim 15, wherein at least one station keeping devicetransmits updated positional information over the acoustic datacommunication channel where the at least one station keeping devicemoves from its known locations.
 18. The method of claim 15, wherein thedevice to locate transmits alert messages over the acoustic datacommunication channel to the station keeping devices.
 19. The method ofclaim 15, wherein at least one station keeping device transmits anupdated acoustic pulse schedule to the device to locate over theacoustic data communication channel.
 20. The method of claim 1, whereina plurality of devices to locate are provided below the surface of thewater.
 21. The method of claim 1, wherein the device to locate includesa display for displaying positional information.
 22. The method of claim21, wherein the device to locate is worn by a diver.
 23. The method ofclaim 21, wherein the display provides direction and distance to a goal.24. The method of claim 21, wherein the device to locate is integratedwith at least one from the group consisting of a heart rate sensor, ablood oxygen level sensor, and a tank pressure sensor.
 25. The method ofclaim 1, wherein the device to locate comprises an autonomous underwatervehicle.
 26. The method of claim 25, further comprising loading theautonomous underwater vehicle with a planned path, the autonomousunderwater vehicle navigating along the planned path based at least uponthe calculated position of the device to locate.
 27. A system fordetermining the position of an underwater device to locate, comprising:a plurality of station keeping devices on or below the surface of thewater, each station keeping device having a means for determining itsposition, a means for keeping time, a transducer for generating acousticpulses in the water, a memory for storing a schedule of acoustic pulses,and a processor in communication with the memory and the transducer fordirecting the generation of acoustic pulses according to the schedule;at least one device to locate, the device to locate comprising a meansfor keeping time, a memory for storing the schedule of acoustic pulsesand a known position for each station keeping device, an acoustictransducer for receiving pulses sent by the station keeping devices, anda processor configured to calculate the position of the device to locateunderneath the water based upon the arrival time of the acoustic pulsesand the scheduled time for generation of the acoustic pulses by theacoustic transducers on the station keeping devices; wherein each meansfor keeping time is synchronized, and each memory stores the sameschedule of acoustic pulses.
 28. The system of claim 27, furthercomprising a plurality of devices to locate.
 29. The system of claim 28,further comprising at least three station keeping devices.
 30. Thesystem of claim 28, further comprising at least four station keepingdevices.
 31. The system of claim 27, wherein the station keeping devicesare buoys located on the surface of the water.
 32. The system of claim31, wherein the means for determining its position on the stationkeeping devices is a GPS receiver.
 33. The system of claim 32, whereinthe means for keeping time on the station keeping devices is also theGPS receiver.
 34. The system of claim 31, wherein the station keepingdevices are subject to movement with respect to their known position,and the microprocessor on the station keeping devices is furtherconfigured to adjust the timing of the transmission of an acoustic pulsewith respect to the acoustic pulse time schedule to account for themovement.
 35. The system of claim 27, wherein the processor in thedevice to locate is further configured to calculate a likely time windowfor the receipt of an acoustic pulse based at least upon a calculatedposition of the device to locate and to filter out pulses that arriveoutside the window.
 36. The system of claim 27, wherein the processor inthe device to locate is further configured to calculate a velocity anddirection of motion of the device to locate based upon at least twocalculated positions of the device to locate.
 37. The system of claim27, wherein the device to locate further comprises a variable gainamplifier and the microprocessor in the device to locate is furtherconfigured to calculate an acoustic pulse attenuation value and adjustthe variable gain amplifier to account for the calculated attenuation ofthe acoustic pulse.
 38. The system of claim 27, wherein themicroprocessor in the device to locate is further configured to measurea local speed of sound value by measuring the time between a scheduledsending of an acoustic pulse by a station keeping device and receipt ofthe pulse by another device over a known distance and the local speed ofsound value is used by the device to locate is subsequently used tocalculate the distance between the device to locate and a stationkeeping device.
 39. The system of claim 27, wherein the device to locateis configured to be worn by a diver.
 40. The system of claim 39, whereinthe device to locate further comprises a display connected to theprocessor for displaying positional information.
 41. The system of claim39, wherein the display is further configured to display direction anddistance to a goal.
 42. The system of claim 27, wherein the device tolocate is integrated with at least one from the group consisting of aheart rate sensor, a blood oxygen level sensor, and a tank pressuresensor.
 43. The system of claim 27, wherein the device to locatecomprises an autonomous underwater vehicle.
 44. The system of claim 43,wherein the memory in the device to locate further includes a plannedpath, the autonomous underwater vehicle navigating along the plannedpath based at least upon the calculated position of the device tolocate.
 45. A device to locate that can calculate its positionunderwater in a system having a plurality of station keeping devices onor below the surface of the water, each station keeping device beinglocated in a known position and including a means for keeping time thatis synchronized with a system time base, a transducer for generatingacoustic pulses in the water, a memory for storing a common systemschedule of acoustic pulses, and a processor in communication with thememory and the transducer for directing the generation of acousticpulses according to the schedule, the device to locate comprising: ameans for keeping time that is synchronized with the system time base; amemory for storing the common system schedule of acoustic pulses and theknown position for each station keeping device; an acoustic transducerfor receiving pulses sent by the station keeping devices, and aprocessor configured to calculate the position of the device to locateunderneath the water based upon the arrival time of the acoustic pulsesand the scheduled time for generating the acoustic pulses by theacoustic transducers on the station keeping devices.
 46. The device ofclaim 45, wherein the processor in the device to locate is furtherconfigured to calculate a likely time window for the receipt of anacoustic pulse based at least upon a calculated position of the deviceto locate and to filter out pulses that arrive outside the window. 47.The device of claim 45, wherein the processor in the device to locate isfurther configured to calculate a velocity and direction of motion ofthe device to locate based upon at least two calculated positions of thedevice to locate.
 48. The device of claim 45, wherein the device tolocate further comprises a variable gain amplifier and themicroprocessor in the device to locate is further configured tocalculate an acoustic pulse attenuation value and adjust the variablegain amplifier to account for the calculated attenuation of the acousticpulse.
 49. The device of claim 45, wherein the microprocessor in thedevice to locate is further configured to measure a local speed of soundvalue by measuring the time between a scheduled sending of an acousticpulse by a station keeping device and receipt of the pulse by the deviceto locate over a known distance and the local speed of sound value isused by the device to locate is subsequently used to calculate thedistance between the device to locate and a station keeping device. 50.The device of claim 45, wherein the device to locate is configured to beworn by a diver.
 51. The device of claim 50, wherein the device tolocate further comprises a display connected to the processor fordisplaying positional information.
 52. The device of claim 50, whereinthe display is further configured to display direction and distance to agoal.
 53. The device of claim 50, wherein the device to locate isintegrated with at least one from the group consisting of a heart ratesensor, a blood oxygen level sensor, and a tank pressure sensor.
 54. Thedevice of claim 45, wherein the device to locate comprises an autonomousunderwater vehicle.
 55. The device of claim 54, wherein the memory inthe device to locate further includes a planned path, the autonomousunderwater vehicle navigating along the planned path based at least uponthe calculated position of the device to locate.
 56. The method of claim20, wherein each of the plurality of devices to locate calculates adistance between itself and a respective station keeping device basedupon the scheduled time for the station keeping device to provide anacoustic pulse and the time that the scheduled acoustic pulse isreceived at each of the respective devices to locate, each device tolocate thereby calculating a distance to a respective station keepingdevice from the same scheduled acoustic pulse.